monado/doc/changes/xrt/mr.1285.md

Introduce @ref xrt_builder struct and various functions on @ref xrt_prober to support them. This is a follow up on the @ref xrt_system_devices changes. These make it much easier to build more complex all in one devices like WinMR and Rift-S. And moves a lot of contextual configuration out of generic drivers like the hand-tracker code needing to know which device it was being used by.